The tiny skull (total length, 17 mm; Supplementary Table 1) of the holotype SMNS 97278 is characterized by an elongate, tapering snout, large orbits and a dome-like skull roof (Fig. 1c). This configuration is typical among Drepanosauromorpha8,16 and superficially resembles that of birds and pterosaurs. The jaws are edentulous anteriorly and bear slender, pointed teeth more posteriorly. Combined with the long, gracile snout, these features of the skull seem appropriate for probing for insects and other small invertebrates from narrow crevices. The dome-like skull roof is predominantly formed by transversely wide frontals and parietals. This resembles the condition in Avicranium, in which the skull roof is part of a highly encephalized skull with a large endocast relative to body size16. As in the drepanosauromorphs Avicranium and Megalancosaurus16,17, the eyes face distinctly forward (Extended Data Fig. 3d). A large unossified opening (fontanelle) is present between the parietals and frontals in SMNS 97278 (Fig. 1b), indicating an early ontogenetic stage for this individual. This is consistent with its diminutive size among the Mirasaura material (crest height is 51 mm versus more than 153 mm in SMNS 97280). The posterior margin of the skull is distinctly inclined posterodorsally (40°) in lateral view (Fig. 1c), a feature shared with Longisquama, Megalancosaurus, Vallesaurus and Avicranium7,17,18,19. The well-preserved left maxilla and the mandible show that antorbital and mandibular fenestrae—classic archosauriform synapomorphies20—are unambiguously absent. The elongate nasal has a ventrolateral process near its posterior end that is separated from the main body by a distinct notch, which forms the posterodorsal corner of the external naris as in other drepanosauromorphs17,19. The jugal lacks a distinct posterior process, and consequently the infratemporal fenestra is open ventrally, as in other early diapsids (including drepanosauromorphs)8,21, most lepidosaurs22 and various Mesozoic marine reptile clades23,24, but in contrast to the fully formed infratemporal bar of most archosauriforms20. The palate and braincase are poorly preserved in SMNS 97278, but short, pointed teeth are unambiguously present on the fragmentary pterygoid remains (Extended Data Fig. 3g). Palatal teeth are a common trait in early diapsids but absent in most archosaurs20.

The vertebral column of the well-preserved postcranial skeleton of SMNS 97279 consists of seven cervical, 24 dorsal and two sacral vertebrae; the ventrally curving tail is incompletely exposed (Fig. 1g). The cervical vertebrae possess hypapophyses and lack ribs, both of which represent drepanosauromorph synapomorphies (Supplementary Information). As in most other drepanosauromorphs, the trunk is long and barrel-shaped, and all dorsal ribs are single-headed. Gastralia are absent. The preserved tail is dorsoventrally deep, but not to the extent seen in some other drepanosauromorphs (for example, Hypuronector)8,25. Except for Longisquama7 and possibly Hypuronector25, Mirasaura shares the presence of a pronounced ‘hump’ in the anteriormost portion of the trunk with other drepanosauromorphs8. The ‘hump’ in Mirasaura is formed by the convex curvature of the vertebral column and a limited dorsal elongation of the neural spines. Its position corresponds to the anterior portion of the integumentary crest, as is evident from SMNS 97279 and SMNS 97278 (Fig. 1 and Extended Data Fig. 1a,d,e). Soft-tissue remains are present in the same region in the holotype of Drepanosaurus unguicaudatus, but although it has been tentatively suggested that they might belong to an integumentary crest as in Longisquama8, the region is insufficiently preserved to determine this with confidence.

The scapulocoracoid of SMNS 97279 is fragmentary, but the scapular blade is clearly dorsoventrally tall and anteroposteriorly very narrow (Fig. 1g and Extended Data Fig. 1e), as in other drepanosauromorphs8. The forelimb lacks derived drepanosaurid features, such as an enormous crescent-shaped ulna and proximal elongation of carpal elements26. The humerus is considerably longer than the radius and ulna and slightly longer than the femur (Supplementary Table 2). The ulna possesses an olecranon process, which is absent in Longisquama7. The ilium is characteristically drepanosauromorph, with an iliac blade that is dorsoventrally taller than anteroposteriorly long and has an anterodorsally extending long axis8. The pes is only partially preserved, but the large, curved unguals and the elongation of the preungual phalanx in digits III and IV indicate an arboreal mode of life for Mirasaura27.

Soft-tissue anatomy and ultrastructure

The integumentary appendages of Mirasaura extend dorsally along the midline of the anterior part of the trunk in SMNS 97278 and SMNS 97279 (Fig. 1a,g). The soft tissues are poorly preserved and evident as impressions with patchy orange staining. In SMNS 97278, the soft-tissue structures are vertically oriented and form a crest of 16 serial, elongated and unbranched integumentary appendages that overlap tightly. The integumentary appendages gradually decrease in height posteriorly, with the anteriormost appendage being both the proximodistally longest and anteroposteriorly narrowest; it is roughly four times the height of the posteriormost appendage (Fig. 1a). SMNS 97279 preserves only the proximal part of the crest, which is closely associated with the ‘hump’ of the vertebral column (Fig. 1g and Extended Data Fig. 1d,e). There is no evidence that individual integumentary appendages are directly associated with specific vertebrae: the bases of the integumentary appendages are not preserved, and there is no evidence of preserved skin or any other soft-tissue structures connecting the appendages and vertebrae. The remaining specimens of Mirasaura represent integumentary appendages without associated skeletal remains. Most of these isolated appendages are organically preserved; the carbon film that defines the structures varies in tone and may be absent locally. The largest isolated crest consists of 20 tightly overlapping integumentary appendages (SMNS 97280; Fig. 1d); the proximal and distal termini are not preserved. The crest has a distinctly and continuously convex anterior margin and a posterior margin that is concave in its distal portion; the posterior portion of the crest is poorly preserved and partially displaced post mortem.

The individual appendages are divided into an anteroposteriorly narrow proximal section with straight margins and a gradually expanding and posterodorsally curving distal portion. The narrow proximal section comprises at least two-thirds of the total length of the anterior appendages but becomes relatively shorter posteriorly. The proximal section is formed by three distinct bands, of which the central band is the widest (Extended Data Fig. 2a,b). The expanded distal section of each individual appendage consists of two laminae that are separated by a distinct medial structure. The laminae are asymmetrical, with the posterior lamina being slightly wider than the anterior lamina (Extended Data Figs. 2g–i and 4). The medial structure can appear as a pair of closely spaced, subparallel dark lines, where the anteriormost of the pair is darker in tone. The medial structure widens gradually towards the distal part of the appendage (Extended Data Fig. 4e). The margins of the laminae are visible as dark lines that are (sub-)parallel to the medial structure. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy analysis shows that dark-toned regions of the integumentary appendages are rich in C (Extended Data Fig. 5). Synchrotron rapid scanning X-ray fluorescence (SRS-XRF) imaging shows that these carbon-rich regions are associated with Fe, Ni and Cu and, to a lesser extent, Mn (Extended Data Fig. 6 and Supplementary Information). These elemental maps also aid in visualizing the medial structure and laminae. The Cu map for specimen SMNS 97280 (Extended Data Fig. 4b) shows that the distal part of the medial structure is evident as a pair of sharply defined parallel lines with striking tonal contrast on either side of each line. Furthermore, analysis of the topography of the appendage surfaces shows that the lines that define the margins of the laminae and the medial structure are regions of high topographic relief (that is, narrow, elevated ridges; Extended Data Fig. 7). The distal ends of the integumentary appendages are not clearly exposed in any specimen: the plane of splitting does not extend to the tips, which are consistently either covered by sediment or fractured and lost from the slab surface. This poor splitting reflects the lack of well-defined lamination of the sediment, which instead shows, at best, crude lamination.

The integumentary appendages are in many (but not all) cases distinctly corrugated (Extended Data Fig. 2). In the proximal portion of the integumentary appendages, a series of curved and/or horizontally oriented rugae (that is, wrinkles or folds) that are roughly equally spaced often occur in the central band. The distal portion of the integumentary appendages exhibits rugae in various orientations, ranging from horizontal to oblique, with oblique rugae oriented in either a lateroventral or a laterodorsal direction (Extended Data Fig. 2g–i). Similar structures are present in the integumentary appendages of Longisquama4,6. The rugae are variable in geometry, distribution and orientation28, and the surface of the appendages, including the rugae, was to some extent malleable, as is clearly indicated by the irregular creases and asymmetrical patterning of the rugae in several specimens (Extended Data Fig. 2a,g). Although distinct and regular compartmentalization or branching through the rugae can thus be excluded, their nature cannot be fully resolved. They might have formed a semiregular external ridging of the appendages in life, as external sculpturing also occurs in scales of pangolins and some extant lepidosaurs29,30. Alternatively, the rugae could plausibly represent taphonomic artefacts resulting from contractional dehydration and/or decay established in the fibrillar keratinous tissue. The presence of rugae in both Mirasaura and Longisquama, despite their distinct preservation as mostly carbonaceous films and as external moulds, respectively, does not exclude a taphonomic origin for the rugae but rather reflects elements of common tissue structure and composition and the timing of specific post-mortem events during diagenesis. The preservation of rugae in Longisquama specifically can be readily explained by diagenetic cementation of the host sediment following contraction of the tissue structure, but before complete decay. Decay of the soft tissues proceeded to completeness only after extensive cementation of the sediment.

SEM imaging of the carbonaceous soft tissues shows rod-shaped to ovoid microstructures, which are preserved either as external moulds or as discrete three-dimensional microbodies that are locally abundant and mutually aligned (Fig. 2a–i) (1,143 ± 207 nm long and 412 ± 97 nm wide; n = 100). The geometry of these microstructures is consistent with that of melanosomes in extant and fossil vertebrates31,32,33,34,35; the geometry is not consistent with that of modern bacteria28,36. These microstructures are therefore interpreted as fossil melanosomes. This interpretation is supported by the carbonaceous composition of the soft-tissue film. The length and width of the melanosomes in Mirasaura differ significantly from those of melanosomes in the skin of extant reptiles and in the feathers of extant birds28 (Extended Data Fig. 8 and Supplementary Data File 13 (available at https://doi.org/10.6084/m9.figshare.27083092)). Importantly, however, the aspect ratio of the melanosomes in Mirasaura differs significantly from that of melanosomes from reptile skin and mammal hair, but not feathers. Aspect ratio is the most important discriminator between melanosomes in reptile skin and mammal hair (where melanosomes have a narrow range of aspect ratio values: that is, less diverse shapes) and feathers (where melanosomes have a wide range of aspect ratio values: that is, more diverse shapes)32. This finding—that the melanosomes of Mirasaura are similarly diverse in shape to those in feathers—is supported by our analysis of melanosome geometry (Fig. 2j and Extended Data Fig. 8). The morphospace of the Mirasaura melanosomes overlaps fully with that of extant and fossil feathers and partly with the morphospace of melanosomes from mammal hair, and only minor overlap is observed with melanosomes from reptile skin. Collectively, these analyses demonstrate that melanosomes in the integumentary appendages of Mirasaura cannot be distinguished morphologically from melanosomes in feathers but differ significantly from those in skin or hair.